The temperature of the object is directly related to the frequency of the light that it emits. At room temperature bodies emit infrared light, but as the temperature increases past a few hundred degrees Celsius, bodies start to emit at visible frequency, from red, through orange, yellow, and white before ending up at blue, beyond which the emission includes increasing amounts of ultraviolet.
Also the amount of radiation emitted by an object increases with temperature difference between the object and its surroundings .
Infrared radiation emitted by an object can be therefore be used to determine the temperature of objects from a distance away. This is called thermography.
Thermography (thermal imaging) is mainly used in military and industrial applications but the technology is reaching the general public nowadays in the form of infrared cameras due to the massively reduced production costs.
Infrared radiation is emitted by all objects based on their temperatures and thermography makes it possible to "see" one's environment with or without visible illumination. A 'false-colour' images is made up by filling in a contour map of temperature boundaries with visible colour.
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